[Tutor] FrozenDict

[Albert-Jan Roskam]

----------------------------------------
> Date: Thu, 8 Oct 2015 11:47:41 +1100
> From: steve at pearwood.info
> To: tutor at python.org
> Subject: Re: [Tutor] FrozenDict
>
> On Wed, Oct 07, 2015 at 04:10:20PM +0000, Albert-Jan Roskam wrote:
>> Hi,
>> I wanted to create a read-only dict to hold some constants. I looked around on the internet and created two implementations:-FrozenDict (derives from collections.mapping)-ChillyDict (derives from dict, which seems more obvious to me)
>> The code can be found here: http://pastebin.com/QJ3V2mSK
>
>> Some questions:1. one doctest from FrozenDict fails: fd.keys() returns
>> an empty list. Why?
>
> No it doesn't.
>
> py> fd = FrozenDict(a=1, b=2)
> py> fd.keys()
> ['a', 'b']
>
>
> That's under Python 2.7.
>
> In 3.3, you will have a problem that FrozenDict is not a proper
> iterator. You can't set self.__next__ = self.next, that won't work.
> Dunder methods have to be on the class, not on the instance, so instead
> of making the assignment in the __init__ method, put this in the body of
> your class:
>
> def next(self):
> # Python 2 method
> ...
>
> __next__ = next # Python 3 method.
>
>
> Unfortunately that's not enough to get it working in Python 3. I need
> more time to think about that.
>
>
>> 2. Is FrozenDict the way to use collections.mapping (aside from the
>> error!). I just discovered this and i seems quite cool (pun intended)
>
> I think the error is quite significant...
>
>
>> 3. Which implementation is better, and why? I like ChillyDict better
>> because it is more straightforward and shorter.
>
> I dislike the ChillyDict implementation because it looks like it
> supports item assignment:
>
> hasattr(ChillyDict, '__setitem__')


Thanks. I had not considered that. I usually use try-except (forgiveness) rather than hasattr (permission)
I will post another implementation shortly.

>
> will return True, but it actually doesn't. That could make it risky in
> code that assumes that the existence of __setitem__ means you can set
> items.
>
>
>
> --
> Steve
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